788 LECTURE XLV. 



plafjrarpus, &c.), and in all cases in which fertilisation is accomplished by means 

 of active or passive motile zoosperms, at least the possibility is given of their 

 meeting with oospheres of more distant origin. Even in the case of Vaucheria, 

 where the antheridium is the sister-cell of the oogonium, the curvature of the 

 former and the direction in which the zoosperms are emitted, points to the fact 

 that fertilisation generally takes place not between the organs which stand next 

 one another, but between those more remote, or even between those of different 

 individuals. 



The tendency to allow only sexual cells of the most different origin possible 

 to fertilise one another within the limits of the same species, is evinced by very 

 various arrangements. The simplest way is by only male or only female organs 

 being produced on each individual of the plant; thus the whole development of 

 the two plants concerned lies between the two sexual cells which come into 

 union, if they originate from the same parent-plant, and a still longer series of 

 developmental processes if the plants in question themselves originate from 

 different parent-plants. Now this distribution of the sexes, which we may designate 

 generally as dicecious, is found in all classes and orders of the vegetable kingdom, 

 and this extension of it alone indicates that it is an adaptation useful for the 

 maintenance of the most different species; thus we find dioecism in many Algae 

 (e. g. most Fucaceaj), in most Characese, many Muscinese, in the prothallia of 

 some Ferns, and most Equisetacese, and further in many Gymnosperms and 

 Angiosperms. 



If the vegetative body which produces the sexual organs is itself large, or 

 at any rate much segmented, a distant relationship of the two kinds of sexual 

 cells is attained by male cells being produced on different branches from the 

 female ones; this case also, which may be denominated generally as 7noncccism, 

 is very widely distributed in the vegetable kingdom (some Algse, many Muscinese, 

 very many Gymnosperms and Angiosperms). 



But even the apparently most unfavourable condition as regards the principle 

 laid down above, that the sexual cells arise close beside one another, is frequently 

 realised in the vegetable kingdom, the sexual cells being thus of close though not 

 always of the closest descent : thus the same cell-filament of the algal genus 

 (Edogonnmi produces male and female cells, the same vesicle of Vaucheria forms 

 antheridia and oogonia close together, the same receptacle ol Fucus platy carpus gives 

 rise to oospheres and zoosperms, the oogonium of most Characese arises close beside 

 the antheridium on the same leaf, the archegonia and antheridia of some Mosses 

 (species of Bryiim) are collected together in one ' flower,' and the prothallia of 

 many Ferns produce both kinds of sexual organs in close proximity to one 

 another. In the flowers of Angiosperms the androgynous sexual apparatus is 

 typical and very general. But in all these cases where the object is apparently 

 to promote the union of sexual cells which are closely allied, mechanisms exist 

 at the same time which prevent the male cells coming into contact with the 

 female ones produced close beside them, or at any rate care is taken that this 

 need not always occur, a fact which was first recognised by Kölreuter (1761) 

 and Conrad Sprengel (1793), and more recently extended by Darwin, Hildebrand 

 and others. It is just in the case of the hermaphrodite flowers and the similar 



